Verified answer

Without knowing exactly what to do, you can't.

Here's a hint: They're replacing the textures with lower resolution versions, among other things.

@Aditya:

I'm sorry, but you're wrong. Even Huffman-coding with a large symbol table won't get the gain you're looking for without discarding some information from the files that are to be compressed.

use kgb archiever..itz vry slow bt it'll cnvrt ur file into around 100 mb......or u cn use peazip it also gr n a bit fast as well